This present application relates generally to methods, systems, and apparatus for monitoring the performance of selective catalytic reduction processes through specie and/or temperature mapping so that the emissions relating to internal combustion engines may be better monitored and/or controlled. More specifically, but not by way of limitation, the present application relates to methods, systems, and apparatus pertaining to performance monitoring of selective catalytic reduction processes through specie and temperature mapping using laser absorption spectroscopy and related processes.
A significant issue related to the use of industrial and utility boiler systems, gas turbine engines, and other internal combustion engines is the amount of nitrogen oxides (or “NOx”) that is released into the atmosphere. As a way to combat this problem, many operators for years have used selective catalytic reduction (or “SCR”) processes to reduce NOx emissions.
As a result, it will be appreciated that selective catalytic reduction processes, as they relate to reducing NOx emissions, are important for protecting and promoting public health. One reason for this is that NOx, when released into the atmosphere, often mixes with other compounds to create smog, which, of course, is a significant form of air pollution in many cities. Accordingly, the Environmental Protection Agency (or “EPA”) sets limits as to the amount of NOx that a facility can legally release into the atmosphere. In order to avoid fines and other penalties, companies that operate such facilities monitor closely and attempt to limit the amount of NOx that is released into the atmosphere.
In general, selective catalytic reduction works by converting nitrogen oxides into diatomic nitrogen (or “N2”) and water (or “H2O”), both of which are harmless and safe for the environment when released into the atmosphere. This chemical reaction (i.e., the conversion of NOx into N2 and H2O) is brought about by combining NOx with a reductant, typically ammonia (or “NH3”), which then comes in contact with the catalyst to produce the reaction that separates the NOx into N2 and H20. When the internal combustion engine, for example, a gas turbine engine, it is operating under steady conditions, SCR systems generally prove very effective at reducing the amount of NOx released. For example, in some applications, NOx emissions may be reduced by up to 90%.
However, during transient operating conditions, for example, engine start-up or load swing conditions, NOx output may spike, which may result in excess NOx (beyond acceptable limits) being released into the atmosphere. Further, in attempting to neutralize these raised levels, conventional systems often over-inject ammonia (i.e., inject an excess amount of ammonia) into the selective catalytic reduction system. This, which is generally referred to as NH3 “slippage”, leads to an equally troubling situation: the release of unacceptably high levels of NH3 into the atmosphere, which may also occasion fines and other penalties against the operator of the combustion engine.
The reasons conventional systems have such difficulty in regulating NOx and NH3 emission levels during transient conditions generally relate to the limitations associated with certain system components, particularly, with the measuring devices used to determine the concentration levels of the relevant compounds in the exhausts, as well as the limiting configuration of the system. These limitations are many. First, gas composition and specie concentration levels are generally measured through time-consuming extractive technologies using heated sample lines. This is a slow process with lag times of many minutes (and, in some cases, hours) and often delivers unreliable results. Second, conventional systems generally lack temperature data in the measurement location. As NH3 absorption rates are dependent on temperature, this data is necessary for precise control of the process. Third, conventional systems lack information regarding the spatial distribution of the relevant compounds through the exhaust. Fourth, conventional systems generally only measure gas composition downstream of SCR.
It will be recognized that, ideally, specific molar match of ammonia to NOx is highly desirable. When this is the case, NOx emissions are reduced as intended while no or little excess ammonia is released into the atmosphere. In practice, as one of ordinary skill in the art will appreciate and for the reasons provided above, this aim has proved to be difficult to achieve. As a result, there is a continuing need for improved methods, systems, and apparatus relating to the monitoring and/or control of selective catalytic reduction processes.